www.youtube.com/watch?v=vprpuc5pLHI&list=PLtaXuX0nEZk8l5Zo038sAhVj1_2T4VCfa&index=9

I watched this video about template override and setup a new template-set called "database".
The template set itsself work. I also created a new css-file called database_style.css in the folder com_k2/templates/database/, but for some reason it's not loaded. I use the Yaml-Template for this work.

If you want to checkout yourself - that's my testpage
test.razen.alfahosting.org/index.php/nahrung

If you want to add a new stylesheet you need to include in your template using Joomla!'s API.
docs.joomla.org/JDocument/addStyleSheet